Prep the pan: Line a 9x13-inch baking pan with parchment, leaving overhang on two sides for easy lifting. Lightly grease the parchment.

Make the shortbread dough: Beat softened butter with granulated and powdered sugars until smooth and creamy, about 1–2 minutes. Mix in vanilla and salt. Add flour and mix just until a soft dough forms.

Press and dock: Press dough evenly into the lined pan. Use the bottom of a measuring cup to level it. Prick all over with a fork to prevent puffing.

Bake the base: Bake at 325°F (165°C) for 20–25 minutes, until edges are lightly golden and the center looks set. Cool in the pan on a rack while you make the caramel.

Cook the caramel: In a medium saucepan, combine butter, brown sugar, condensed milk, syrup, and salt. Cook over medium heat, stirring constantly, until it comes to a gentle bubble. Reduce heat to medium-low and cook 5–7 more minutes, stirring nonstop, until thickened and slightly darkened. It should coat the spatula and leave trails.

Add vanilla and pour: Remove from heat, stir in vanilla, and immediately pour the caramel over the cooled shortbread. Tilt the pan and smooth the surface quickly.

Set the caramel: Let the caramel cool at room temperature 20–30 minutes, then chill 20 more minutes until firm to the touch but not rock hard. This helps the chocolate set cleanly.

Melt the chocolate: In a microwave-safe bowl, heat chocolate with oil in 20–30 second bursts, stirring between each, until smooth. Or melt over a double boiler, keeping water at a simmer and steam away from the bowl.

Add the chocolate layer: Pour melted chocolate over the caramel. Spread evenly with an offset spatula. Tap the pan gently on the counter to pop any bubbles. Sprinkle flaky salt if using.

Set and slice: Let the chocolate set at room temp until just firm, about 45–60 minutes, or chill 15–20 minutes. Use the parchment to lift the slab out. For clean slices, warm a sharp knife under hot water, wipe dry, and cut into bars or fingers.